1. Field of the Invention
The present invention relates to a method of controlling solder deposition on a base metal, and in particular, to a method comprising the use of two distinct fluxes to bond a solder to a base metal or substrate and facilitate the removal of residues.
2. Description of the Prior Art
When soldering to a metal surface, whether the purpose for soldering is to create electrical connections or non-electrical, mechanical connections, the solder must metalize, or bond to, the metal surface. Good electrical or thermal conduction by the solder is dependent on void-free solder bonding to the metal surface. An example in the electronics assembly industry is to solder a metallic lid or cover onto an electronic device that must be cooled. The heat conducting lid can then radiate heat or transfer it to a thermal pipe for removing the heat, thereby cooling the device. The soldering flux must be selected for its ability to remove contamination, mostly oxides, from the metal surface so the melted solder can properly bond to the metal.
The vast number of soldering fluxes can be placed into groupings or categories by the corrosive nature of their residues as is done in IPC/EIA Standard J-STD-004 xe2x80x9cRequirements for Soldering Fluxesxe2x80x9d. This industry document classifies fluxes according to their basic composition and percent halides included in the composition. Another industry standard is ASTM B 32 xe2x80x9cStandard Specification for Solder Metalsxe2x80x9d that includes a similar grouping of flux types. Additionally, another international standard is ISO 9454 xe2x80x9cSoft Soldering Fluxesxe2x80x94Classification and Requirementsxe2x80x9d that delineates the performance requirements for fluxes classified by ingredients. There may be flux choices not specifically covered by these standards, but generally the fluxes can be categorized by composition into three groups and defined as:
Inorganic Fluxxe2x80x94A solution of inorganic acids and/or salts, including, but not limited to, halide salts of metals, such as zinc chloride, zinc bromide, stannous chloride, stannous bromide, stannous fluoride, sodium chloride, in water and optionally containing ammonium chloride, mineral acids, such as hydrochloric acid, hydrobromic acid, phosphoric acid.
Organic Fluxxe2x80x94Primarily composed of organic materials other than rosin or resin, including, but not limited to, water soluble carboxylic acids, such as formic acid, acetic acid, propionic acid, malonic acid, glycolic acid, lactic acid, glyceric acid, malic acid, tartaric acid, and citric acid; water insoluble carboxylic acids, such as stearic acid, oleic acid, benzoic acid, salicylic acid, succinic acid, adipic acid, azelaic acid; optionally containing in admixture amines, amides, and hydrohalide derivatives of the amines and acids.
Rosin Fluxxe2x80x94Primarily composed of natural resins extracted from the oleoresin of pine trees and refined, the composition may also contain additives to increase activity, such as other organic acids and amine hydrohalides. Rosin fluxes are generally not water soluble.
Additionally, the flux compositions may vary in activity as indicated by the level of halides included in the flux and by corrosion testing. Inorganic and Organic fluxes are generally water-soluble, while rosin fluxes are solvent-soluble. It is not the intention of the present invention to specify a flux type or composition, but rather to demonstrate the potential use of the variety of available fluxes.
High activity fluxes, in particular the Inorganic and Organic water-soluble types, are very effective for soldering even the most difficult metals, but may cause the formation of harmful, insoluble, corrosive residues on the soldered assembly. If allowed to remain on the substrate, the residues can result in electrical or mechanical failure of the product. In order to use a high activity flux, the present invention utilizes a second flux to render the residues from the high activity flux soluble in water, or other suitable solvent, so they can be removed from the product by washing. The second flux is a finishing flux that may or may not contain halides or other corrosive materials. If the finishing flux contains corrosives and/or halides, they must be cleanable after the reflow process. These fluxes are used in the method of the present invention with a preform of any solderable composition because they allow for versatility in depositing solder of any size or shape to the substrate.
Prior art processes to reduce the amount of residue formed on the metal after applying a deposition of solder on a base metal include using a less active flux, which can result in poor solder joints, dewetting or incomplete soldering. Another method utilizes a resist material to define the area of deposit on the metal substrate. The flux is applied and placed in the area bounded by the resist and, and then the metal substrate is dipped in molten solder or passed across a wave of solder. This method is undesirable because of the potential heat damage on the substrate and irregular solder deposit. Further, another prior art method utilizes a soldermask in an attempt to limit the spread of the solder deposit when solder paste is applied in the area defined by the soldermask. Though the solder deposit can be more uniform. The use of a soldermask would generally be expensive, time-consuming and inefficient.
The present invention utilizes two different fluxes for bonding preformed solder onto a base metal wherein the base metal or substrate, preform and flux may be subjected to reflow conditions one or two times.
The present invention is generally directed to methods of controlling solder deposition on a base metal or substrate using conductive connections; the connections may be for applications involving the passage of electrical or thermal energy. The methods may be utilized with electrical connections and non-electrical connections, i.e., transporting thermal energy from the device via heat sinks. The methods may also be used in forming connections for the transporting of electrical energy from one conductive metal to another metal. The first method comprises applying a sufficient amount of an attaching flux to a base metal or substrate, placing a preform on the flux on the substrate, applying a sufficient amount of finishing flux onto the preform, subjecting the substrate, fluxes, and preform to reflow conditions, cooling, cleaning the substrate and preform, now the solder deposit, and drying the substrate and solder deposit. An alternate method of the present invention comprises applying the attaching flux to a base metal or substrate, placing a preform on the flux on the substrate, subjecting the substrate, attaching flux, and preform to reflow conditions, then applying a finishing flux to the substrate and preform, now the solder deposit, subjecting the substrate, solder deposit, and finishing flux to reflow conditions, cooling, cleaning the substrate and solder deposit, and drying.
It is the object of the present invention to provide a method of controlling solder deposition on a base metal.
It is another object of the present invention to provide a method of controlling solder deposition on a base metal used in a conductive connection.
It is an object of the present invention to provide a method of controlling solder deposition on a base metal wherein the base metal is used for the passage of electrical or thermal energy.
It is an object of the present invention to provide a method of controlling solder deposition on a base metal that forms a bond with a heat sink, electrical connection or non-electrical connection.
It is an object of the present invention to use an aggressive attaching flux and a compatible finishing flux to provide a reliable bond when soldering metals to substrates.
It is an object of the present invention to provide a method of using an aggressive flux with a solder without regard to the formation of harmful, insoluble, corrosive residues.
Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein given the scope of the present invention, as defined by the claims.